Most animals of the deep are of ordinary size. The ferocious-looking large-fanged viperfish, for example (see Midwater page), is typically only a few centimetres to 30 cm long. But an unusual feature of some animals of the deep is known as gigantism: a larger body size than their comparable relatives in shallower waters. For example:

While we understand why hydrothermal-vent tubeworms can be enormous- they live off of hydrothermal vents which supply practically unlimited energy for bacterial growth- we are not sure how and why some other animals (inhabiting often energy-poor habitats) can get so large.